Switching power supply source

ABSTRACT

A switching power supply source including an inductance with first and second terminals; an output node; an NMos transistor, the drain of which is connected to the first terminal; a PMos transistor, the drain of which is connected to the first terminal; a control device generating control signals for NMos and PMos transistors assuring that these transistors are not conducting simultaneously; a capacitor with a third terminal connected to the first terminal and a fourth terminal; a resistance with a fifth terminal connected to the fourth terminal and a sixth terminal; and an NMos transistor the drain of which is connected to the grid of the PMos transistor and the gate of which is connected to the fourth terminal.

BACKGROUND OF INVENTION

1. Field of Invention

The invention relates to switching power supply sources in general andparticularly switching power supply sources with an inductance for whichthe voltage level is defined by two control switches.

2. Discussion of Related Art

FIG. 1 illustrates a switching power supply source 1 according to thestate of the art. This switching power supply 1 is a DC-DC step upvoltage converter like that described in document FR-2 841 061. Theswitching power supply source 1 has an inductance L connected firstly toa DC power supply source Vbat and secondly to an intermediate node Lx.The source of an NMos transistor N1 is connected to the ground and itsdrain is connected to the node Lx. The drain of a PMos transistor P1 isconnected to the node Lx and its source is connected to the output Vsfrom the switching power supply 1. A load R to be powered and acapacitor C are connected in parallel between the output Vs and theground. A switching power supply control device 2 is connected to thegates of the transistors P1 and N1. The control device 2 controlstransistor P1 and N1 such that one is blocked (non-conducting) when theother is conducting, so as to prevent current passing through theinductance becoming negative. To prevent transistors P1 and N1 fromconducting simultaneously when one transistor is blocked, the othertransistor is held blocked for a period usually between 20 and 100 nsbefore it is made conducting.

During the transient phase in which the two transistors are blocked, theenergy stored in the inductance is dissipated in the substrate throughthe parasite PNP transistor intrinsically formed by transistor P1 whenusing CMOS technology. This current dissipation into the substratereduces the energy efficiency of the switching power supply.Furthermore, this current can disturb operation of other devicesintegrated into the substrate. Moreover, when the parasite PNPtransistor formed by P1 is conducting, the intermediate node Lxexperiences an overvoltage that can reduce the reliability of thetransistor N1. The invention is intended to solve one or several ofthese disadvantages.

SUMMARY OF INVENTION

The invention thus relates to a switching power supply sourcecomprising:

an inductance with first and second terminals;

an output node;

an NMos transistor, the source of which is connected to the ground andthe drain of which is connected to the first electrode of theinductance;

a PMos transistor, the drain of which is connected to the firstelectrode of the inductance;

a switching power supply control device generating switching controlsignals for NMos and PMos transistors capable of assuring that thesetransistors are not conducting simultaneously;

a capacitor with a first terminal connected to the first terminal of theinductance and a second terminal;

a resistance with a first terminal connected to the second terminal ofthe capacitor and a second terminal connected to the ground;

a detection NMos transistor, the source of which is connected to theground, the drain of which is connected to the gate of the PMostransistor, and the gate of which is connected to the second terminal ofthe capacitor.

According to one variant:

the second terminal of the inductance is connected to a DC power supplysource;

the source of the PMos transistor is connected to the output node.

According to another variant, there is at least one other output nodeand at least one other PMos transistor, the drain of which is connectedto the first electrode of the inductance, the source of which isconnected to the other output node and the gate of which is connected tothe drain of the detection NMos transistor.

According to yet another variant, the switching power supply source alsocomprises a blocking transistor with a first electrode connected to theground, a second electrode connected to the gate of the detection NMostransistor and a control electrode; the switching power supply controldevice making the blocking transistor conducting when the detection NMostransistor is conducting.

According to another variant, the switching power supply source alsocomprises a control transistor with a first electrode connected to theground, a second electrode connected to the gate of the PMos transistorand a control electrode; the switching power supply control devicecontrolling simultaneous conduction of the control transistor and theblocking transistor.

The transistors, the capacitor and the resistance may be integrated onthe same substrate.

BRIEF DESCRIPTION OF DRAWINGS

Other characteristics and advantages of the invention will become clearafter reading the description given below for information and that is inno way limitative, with reference to the attached drawings in which:

FIG. 1 illustrates a voltage step up switching power supply according toprior art;

FIG. 2 illustrates an example voltage step up switching power supplyaccording to the invention;

FIG. 3 shows a timing diagram representing operation of the switchingpower supply to make its PMos transistor conducting.

DETAILED DESCRIPTION

The invention detects blocking switching of the NMos transistor and tomake the PMos transistor conducting when this detection occurs. Blockageof the NMos transistor creates an overvoltage at the PMos and NMostransistor drains. An RC circuit connected to the drains of the PMos andNMos transistors makes detection NMos transistor conducting when theovervoltage occurs. When the detection transistor is conducting, thePMos transistor is also made conducting.

The invention assures conduction of the PMos transistor as soon as theblockage of the NMos transistor is detected. Thus, the time during whichthe NMos transistor and the PMos transistor are blocked simultaneouslyis particularly low. Consequently, the conduction time in the substratethrough the parasite transistor is also low, which increases theefficiency of the switching power supply and limits any disturbances toother integrated circuits. These results are also obtained withoutmaking use of a comparator circuit with a low response time, which wouldincrease the cost and difficulties of integrating the switching powersupply.

FIG. 2 shows a switching power supply source 3 forming a voltage step upDC-DC converter. An inductance L has a first electrode connected to anintermediate node Lx and a second electrode connected to a DC powersupply source Vbat. An NMos transistor N1 has its source connected tothe ground and its drain connected to the intermediate node Lx. A PMostransistor P1 has its source connected to an output node Vs and itsdrain is connected to the intermediate node Lx. The first terminal of acapacitor C1 is connected to the intermediate node Lx and its secondterminal is connected to the grid of an NMos transistor N2. The sourceof transistor N2 is connected to the ground and its drain is connectedto the grid of transistor P1. A diode D1, a resistance R1 and atransistor T1 are connected in parallel between the grid of thetransistor N2 and the ground. A transistor T2 has a first electrodeconnected to the ground and a second electrode connected to the grid oftransistor P1. A switching power supply control device 2 generatescontrol signals gp and gn. These control signals are designed to controlswitching of transistors N1 and P1 so as to define the voltage level onthe output node Vs, while preventing the transistors N1 and P1 frombeing conducting simultaneously. The signal gn is applied to the grid oftransistor N1 and the signal gp is applied to the control electrodes oftransistors T1 and T2.

Operation of the switching power supply 3 when the transistor N1 isblocked is illustrated in FIG. 3.

Before time t1, transistor N1 is conducting, its grid being polarized bythe signal gn at high level. The signal gp is low and thus transistorsT1 and T2 are blocked. Transistor N2 is blocked by the low level appliedto its gate (signal GN2). Transistor P1 is blocked by the high levelapplied to its gate (signal GP1).

At time t1, the control device 2 generates a falling front of the signalgn, blocking the transistor N1. An overvoltage is then generated on theintermediate node Lx. A current then passes through the capacitor C1 andthe resistance R1. The voltage GN2 then increases and makes thetransistor N2 conducting. The voltage Gp1 then drops and the transistorP1 becomes conducting. Thus, an overvoltage on the intermediate node Lxmakes the transistor P1 conducting.

A rising front of the signal gp is generated at time t2, so as to blockthe transistor N2 quickly while transistor P1 remains conducting.Transistors T1 and T2 are thus made conducting. The grid of N2 is thenconnected to the ground and the transistor N2 blocks. The transistor T2that is conducting keeps transistor P1 conducting.

The inventors have determined that the invention provides a means ofobtaining a time shorter than 5 ns between when N1 is blocked and whenP1 is made conducting. During the period between when P1 is madeconducting and time t2, the power dissipated in transistor N2 issignificantly less than the power dissipated in the substrate accordingto prior art. The efficiency of the switching power supply is thusimproved.

The invention is particularly suitable for a switching power supplysource with several outputs. Thus, it will be possible for the voltagestep up converter illustrated above to have several output nodes. Eachoutput node is connected to the source of a corresponding PMostransistor, the drain of this transistor being connected to theintermediate node Lx. The drain of transistor N2 is connected to thegrid of each PMos transistor. Thus, an overvoltage on the intermediatenode Lx makes all PMos transistors associated with the output nodesconducting.

Those skilled in the art will note that the solution according to theinvention can be implemented with standard components, which makes iteasy to integrate them into the same substrate as the switching powersupply transistors.

Although the example described deals only with a voltage step upconverter, those skilled in the art could easily adapt it to create avoltage step down converter based on the description given above.

Having thus described at least one illustrative embodiment of theinvention, various alterations, modifications, and improvements willreadily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be within the scope ofthe invention. Accordingly, the foregoing description is by way ofexample only and is not intended as limiting. The invention is limitedonly as defined in the following claims and the equivalents thereto.

1. Switching power supply source comprising: an inductance with firstand second terminals; an output node; an NMos transistor, a source ofwhich is connected to a ground and the drain of which is connected to afirst terminal of the inductance; a PMos transistor, a drain of which isconnected to the first terminal of the inductance; a switching powersupply control device generating switching control signals for NMos andPMos transistors capable of assuring that these transistors are notconducting simultaneously; a capacitor with a first terminal connectedto the first terminal of the inductance and a second terminal; aresistance with a first terminal connected to the second terminal of thecapacitor and a second terminal connected to the ground; and a detectionNMos transistor, a source of which is connected to the ground, a drainof which is connected to a gate of the PMos transistor, and a gate ofwhich is connected to the second terminal of the capacitor.
 2. Switchingpower supply source according to claim 1, wherein the second terminal ofthe inductance is connected to a DC power supply source; the source ofthe PMos transistor is connected to the output node.
 3. Switching powersupply source according to claim 2, further comprising at least oneother output node and at least one other PMos transistor, a drain ofwhich is connected to the first electrode of the inductance, a source ofwhich is connected to the other output node and the gate of which isconnected to the drain of the detection NMos transistor.
 4. Switchingpower supply source according to claim 1, further comprising: a blockingtransistor with a first electrode connected to the ground, a secondelectrode connected to the gate of the detection NMos transistor and acontrol electrode; the switching power supply control device making theblocking transistor conducting when the detection NMos transistor isconducting.
 5. Switching power supply source according to claim 4,further comprising: also comprising a control transistor with a firstelectrode connected to the ground, a second electrode connected to thegate of the PMos transistor and a control electrode; the switching powersupply control device controlling simultaneous conduction of the controltransistor and the blocking transistor.
 6. Switching power supply sourceaccording to claim 1, wherein the transistors, the capacitor and theresistance are integrated on the same substrate.